1. Field of the Invention
The invention relates to a mobile telecommunications system including a fixed network comprising mobile exchanges, base stations for mobile stations, and cordless telephone base stations connected to and under control of said mobile exchanges. In particular, the present invention relates to a method for performing handover from the base stations for mobile stations to the cordless telephone base stations.
2. Description of the Related Art
Conventional cordless telephones operate as an extension to the Public Switched Telephone Network (PSTN). A cordless telephone system consists of a portable handset and a base station (a fixed part) connected to PSTN. The first cordless telephone systems were analog systems, such as CT1. Recently digital cordless telephone systems, such as CT2 and DECT (Digital European Cordless Telephone) have been introduced. When a cordless telephone is a subscriber of PSTN, it is accessed by dialing the telephone number associated with the fixed subscriber connection to which the base station is connected. Also the subscriber has access to the PSTN only via the home base station. In CT2 and DECT, also a telepoint service is introduced, whereby a subscriber to the service can make calls away from home via a public base station. Also a common air interface for cordless telephones was introduced to facilitate roaming between systems and to improve service coverage.
There has also been a proposal to combine a cordless telephone system with a cellular mobile telecommunications system to further improve the roaming capabilities by utilizing the mobility control of the cellular network. When combined with the mobile telecommunications systems, the roaming of the cordless telephone handsets within the cordless telephone systems is fully supported. However, the cordless telephone handset is not able to utilize the more extensive radio coverage of the supporting cellular network or to perform handover to or from the cellular network. This disadvantage is due to several reasons.
Firstly, the radio interfaces of cordless telephone systems and the mobile radio systems are usually incompatible.
Secondly, the handover procedures are different. In cordless telephone systems, the handover procedure is usually initiated by the portable terminal. More particularly, when a cordless telephone terminal considers it necessary to change from one base station to another, the terminal scans the frequency band and selects the appropriate base station for the handover. In most of the mobile telephone systems the handover is controlled by the network. The mobile station only measures the quality of the radio connection and forwards the measuring results to the fixed network. One of the network elements, for example a mobile exchange, makes a decision for handover on the basis of the obtained information.